While I think this may come across as a "stupid question" I need to ask for support to close a slight debate between myself and another. 

When joining two different thicknesses of 2205 SS - in a joint configuration similar to TC-U5-GF and utilizing one plate having a thickness of .625" containing all the bevels and another plate having a thickness of 1.50" without bevel. What would be the proper welder qualification range to be qualified to weld this joint? That based on the thinner material .625"  ( 2T) or that based on the thicker material 1.50" (unlimited)?

I believe the thinner material is the proper material thickness to consider based on the fact that all effective throat, weld size and most penetration is within this material.